1. Field of the Invention
The present invention relates to a method and to an apparatus for automatic inspection of moving surfaces, in particular to a method and an apparatus for automatic inspection of moving surfaces for applications such as the inspection of steel strips, wood, leather or tiles. Even more particular the present invention relates to a method and an apparatus for automatic inspection of moving surfaces using at least three different illumination/observation channels.
2. Description of Prior Art
Products like the above-mentioned steel strips, wood, leather or tiles are typically produced at high speed in a continuous process and they have to be inspected during motion. The defects which have to be detected and classified automatically are anomalies in the surfaces with respect to e.g. reflectivity, color, glossiness, texture and the 3D-profile of the surface under inspection.
In the prior art, systems for automatic surface inspection are well established and used for industrial applications such as the inspection of steel, tiles or wood. The applied cameras are monochrome or color line scan cameras. For illumination fluorescent lamps, halogen lamps or fiber optic illuminators are commonly used. The defects to be detected and classified are e.g. scratches, dents, knots and the like as defined by the application. Typically these defects manifest themselves in different ways, e.g. in deviations of reflectivity, glossiness, color, texture or the 3D-profile of the surface under inspection.
A critical part in the system design is to define the apparatus for image acquisition, including the selection of a camera and the illumination system, and to determine the geometrical relations of the components. The aim is to achieve images from the surface which contain the necessary information to detect and to distinguish all of the defects automatically, including 3D-defects. In many cases the result is disappointing. The reason is simply that monocular images do not contain reliable and unambiguous information on the 3D-profile and glossiness of the surface. To overcome this problem a multi-camera setup is used in some applications by means of which the surface is inspected under different viewing conditions simultaneously. In such setups, typically two monochromatic cameras and one illuminator are used to obtain bright and dark field images of the same object.
These setups suffer from a number of shortcomings: the alignment of two line scan cameras is difficult, and the mechanical constructions for these systems become heavy.
In the article by R. J. Woodham, "Photometric Method for Determining Surface Orientation from multiple Images", in Optical Engineering Vol. 19, 191, pp. 139 to 144, 1980, a photometric stereo technique is described. The principle of this technique is to take multiple images from the same object and with the same camera, and to vary the direction of the incident illumination between successive images, while holding the viewing geometry constant. This provides sufficient information to determine the surface orientation, i.e. to gather 3D-information, of each surface element of the inspected object at each image point.
The technique is named photometric stereo because it uses the radiance values recorded at a single pixel location in successive views, rather than the relative positions of displaced features in binocular stereo. Since the viewing geometry in this technique is not changed, the correspondence between pixels in the taken set of images is known a priori.
The photometric stereo method requires that the inspected object is resting, giving time to switch the lamps and to take the images, and that the reflectance distribution of the surface is known. Therefore, the method cannot be applied for inspection of moving materials and surfaces of unknown reflectivity.
DE 195 11 534 A1 relates to a method and an apparatus for detecting 3D-defects, such as dents and steps in a flat surface, in applications for automatic surface inspection, following the idea of photometric stereo. The surface under inspection is simultaneously illuminated with at least two lamps from different directions under dark field conditions, where the light from the lamps has different colors. A color line scan camera is used for image acquisition and 3D-defects are detected by analyzing the measured color values.
This method yields information on 3D-defects but not on glossiness and reflectivity of the surface because a symmetrical dark field illumination is used. Therefore, the capabilities for discriminating between different types of defects are limited. Furthermore, defect detection is done by a color classifier. Using this method it is not possible to adapt to a changing appearance of the inspected surface or to changes of the illumination. In practical applications, the sensitivity for defect detection is limited.
EP 0 764 845 A2 describes an apparatus for image acquisition which is similar to the one described in DE 195 11 534 A1, but the method for the detection of 3D-defects is only based on shadows, which can be observed at the edges of steps in the surface.
In the article by M. Magee et al., "Identification of Flaws in Metallic Surfaces Using Specular and Diffuse Bispectral Light Sources" in SPIE, Vol. 1825, Intelligent Robots and Computer Vision XI, pp. 455 to 468, 1992, a method for identification of flaws in metallic surfaces is described. The aim of this method is to enhance contrast and detection rate for scratches during automatic surface inspection of resting cast metal parts. The object under inspection is illuminated with light of different colors: there is one channel for a dark field using a very shallow angle of incidence and one channel for bright field. The used effect is that the light from the shallow dark field illumination will be scattered by sharp edges of the scratches and therefore scratches will look dark in the bright field and bright in the dark field. A second channel for dark field illumination is missing and it is therefore not possible to estimate the slope of surface elements on surfaces with varying glossiness and reflectivity.